Various drive force distribution devices have been proposed. It is conceivable that a structure in which a first roller that rotates together with a rotating member constituting a torque-transfer path toward main drive (road-)wheels and a second roller that rotates together with a rotating member constituting a torque-transfer path toward auxiliary drive wheels are in contact with each other to be pressed radially to each other is employed by using a traction transmission method as disclosed in Patent Literature 1.
In this traction-transmission-type drive force distribution device, a torque obtainable for the main drive wheels can be partly distributed and outputted to the auxiliary drive wheels by a traction transmission generated at a radially-pressing contact portion between the first roller and the second roller. Thereby, drive force can be distributed and outputted to the main drive wheels and the auxiliary drive wheels.
The above drive force distribution device needs a traction transmission-capacity control for controlling a traction transmission capacity of the drive force distribution device, i.e., for controlling a traction transmission capacity at the radially-pressing mutual-contact portion between the first roller and the second roller, to bring the traction transmission capacity to a torque capacity according to a required distribution drive force for the auxiliary drive wheels.
In the traction transmission-capacity control proposed by the Patent Literature 1, it is stated that a mutual radially-pressing force between the rollers automatically attains a traction transmission-capacity value according to a transfer torque.